This invention relates to the field of injection blow molding. More specifically, it relates to a method for obtaining a higher degree of biaxial orientation of articles made by such processes, and may have application in the field of either blow molding or vacuum molding. Prior designs for injection blow molding machines have primarily related to designs in which minimum machine cycle time could be obtained. For example, U.S. Pat. No. 2,853,736 teaches the formation of a parison at a first injection station and immediate transfer thereof to a second station at which they are blown to desired configuration with subsequent ejection of the blown article at a third station. While such apparatus may be sufficient and economical for the manufacture of many articles, they are not believed to result in a high strength, oriented article such as a bottle suitable for containing pressurized liquids or beverages.
Further, U.S. Pat. No. 3,492,387 suggests that a thermoplastic material may be injected into an injection mold; that the slug of thermoplastic material is to be held in the mold for a time period of four seconds to permit cooling, with subsequent expansion of the thermoplastic slug by mechanical means; such procedures intended to result in a more highly oriented hollow article. Finally, U.S. Pat. No. 3,311,684 suggests that a parison may be extruded, with the interior walls of said parison being cooled by a fluid, followed by stretching of the parison in vertical and in radial directions so as to obtain the desired biaxial orientation.
Although U.S. Pat. No. 3,492,387 teaches orientation, it is not believed that the apparatus is best suited for the formation of containers for pressurized liquids or achieves the desired strength. Alternatively, U.S. Pat. No. 3,311,684 involves an extrusion process with its resulting waste, and does not seemingly provide for uniform heat distribution within the parison whereby maximum orientation can be achieved.